CAT converters must primarily conform to the law, that is, they must perform their function to ensure that toxic exhaust emissions has been remove to a certain degree before releasing them in public air.
Upgrading these is primarily a question of investigating what the market offers as an upgrade. A high performance CAT or a competition CAT can flow significantly more than a stock CAT. However, CAT flow performance of highly tuned cars must be checked regularly, as many do not hold up well to extended wide-open throttle. A collapsed CAT can easily rob 5-10% of power output.
It is not just the honeycomb innards of the CAT that can cause flow restriction. Even when the honeycomb is in good condition, a poorly designed CAT will impede gas flow owing to turbulence cause by the exhaust diverging at anything from 35 degrees to 60 degrees into the honeycomb, then converging again into the tailpipe at a similarly steep angle.
To overcome this problem the CAT must have a gently entry and exit taper. Notice the difference between the two images, and their entry and exit angles (marked in red). The less the taper is of the entry and exit, the greater the flow.
On the inlet side a taper of 10-12 degrees allows the exhaust gas to expand gradually to the full size of the honeycomb with minimal turbulence. On the exit side a taper of 12-15 degrees forces the gases to converge into the tailpipe without unduly disturbing the exhaust flow.
If an upgraded CAT is not an option, a possible rewelding of the entry and exit taper can be done to increase the flow of the stock unit. This can result in double the flow, however care should be taken in not damaging the honeycomb innard.